The present invention relates generally to a Terahertz Optical Asymmetric Demultiplexer (TOAD) and, more particularly, to a TOAD having an enhanced extinction ratio; i.e., that cleanly closes its switching window, avoiding inadvertent switching of signals after the switching window has closed.
Different TOAD configurations have been described in various patents, patent applications and other publications. For example, a Sagnac-loop TOAD is described in U.S. Pat. No. 5,493,433, a Mach-Zehnder TOAD is described in 5,825,519, and a Michelson TOAD is described in pending application Ser. No. 09/612,427, filed Jul. 7, 2000, pending. Generally, a TOAD is an optical device having a data signal input port, one or more control signal input ports, one or more non-linear elements, each having a first state in which a characteristic (such as phase) of the light passing through it has a first value and a second state in which a characteristic of the light passing through it is changed to a second value, and an output port. The data input port, the control input port or ports and the output port are coupled to the one or more non-linear elements such that at least two optical paths are formed from the data input port to the output port, each including a non-linear element. One or more control signals input at the control input port or ports cause the non-linear element in each path to change from its first state to its second state. An optical data signal input to the data input port will produce an output data signal on the output port if light on the two optical paths arrives at the output port having passed through different non-linear element states. The optical paths may include the same non-linear element or elements or different elements. If the light signals on the two optical paths do not pass through different non-linear element states, substantially no output data signal is produced on the output port.
In TOADs using more than one non-linear elements, such as the Mach-Zehnder TOAD and Michelson TOAD, current TOAD configurations create a switching window by first saturating the non-linear element in one optical path and then, at a time xcfx84 later, saturating the non-linear element in the other optical path. Saturating a non-linear element causes it to change from its first state to its second state. Only data signals passing through the first and second non-linear elements during the window of duration xcfx84 when the first non-linear element is saturated and the second non-linear element is not saturated will be switched to the output port. One drawback of such current TOAD configurations is that since the first and second non-linear elements are saturated at different times, they decay to their non-saturated states at different times, leading to residual leakage of light through the output port outside the switching window. The maximum extinction ratio of such TOADs (i.e., the ratio of the output power from the switch in its off state to the output power in its on state), which is ideally as close to zero as possible, may thus be higher than desired. This causes, for example, a crosstalk penalty when the device is configured as a demultiplexer in an OTDM (Optical Time Division Multiplexing) network, i.e., light may leak to the output port after the switching window has closed into subsequent time slots.
In addition, certain TOAD configurations, such as the forward-pumped Mach-Zehnder TOAD (shown in FIGS. 1 and 2 of U.S. Pat. No. 5,825,519) and the Michelson TOAD (shown in pending application Ser. No. 09/612,427, filed Jul. 7, 2000, pending), require four ports: a data port, two control ports, and an output port. It would be advantageous to reduce the number of ports to facilitate integration of such devices and to reduce the complexity of packaging such devices, including the coupling of optical fibers to the wave guides.
It is therefore an object of the present invention to provide an enhanced extinction ratio for TOADs having at least one unique non-linear element in each of its at least two optical paths from its data input port to its output port.
It is a further object of the present invention to reduce the number of ports required by certain Mach-Zehnder and Michelson TOADs.
Briefly, a TOAD in accordance with the present invention has preferably two non-linear elements (NLEs). A data signal is input on one port and is split onto two optical paths, each including one NLE. The optical paths converge at an output port. To start a switching window, a first control signal is input on an optical path that includes only one of the two NLEs. To close a switching window, one or more control signals are input such that both NLEs receive a control signal at a predetermined time after the first control signal is received by one of the NLEs. Only data signals passing through the first NLE during the switching window are output on the output port.
Since both NLEs receive a second control signal at the same time, they will decay together and thus avoid creation of unintended switching windows.
Additionally, in certain embodiments of the present invention, the second control signal can be input on the data port or the output port, eliminating the need for a separate port for the second control signal, as required in certain known TOADs.